The nerve sensor inside your rectum that causes you to feel an increasing urgency to poop the closer you get to a toilet. The proximity sensor is activated by an already present need to take a crap. Once in the vicinity of a restroom, your proximity sensor will alert you that you are running out of time to reach the toilet and drop your pants, or suffer the consequences...

As I walked towards the bathroom, cheeks tightly clenched, my proximity sensor went crazy and I nearly pooped my pants before I could sit down.